The present invention relates to the selective determination of a fuse failure in situations where the fuses being monitored are fixed to a rotating device. An example of this situation is a brushless exciter used with an electrical generator. Brushless exciters typically have electrical circuits consisting of many fused diodes, the conductivity of which are important to the operation of the exciter and corresponding generator.
The basic function of a brushless exciter is to convert three phase alternating current to direct current which provides the rotating field of the alternating current turbine generator. This is accomplished by rectifying the alternating current with diode circuits mounted on the rotor of the brushless exciter. For a three phase rectifier circuit, at least six diodes are required and in situations where very high currents are anticipated, multiple diodes are applied in parallel to reduce the current in any individual diode. This procedure produces many parallel current paths within the total circuit with each path requiring a fuse to protect its associated diode.
When multiple parallel paths exist, the failure of a single fuse in one of them may not produce a sufficient overload on the components in paths parallel to it since, in exciters for large generators where a relatively large number of diodes is necessary to carry the current, an excess number of diodes is normally provided so that some predetermined number of diodes can be nonfunctional without seriously overloading the remaining functional diodes. For example, in a design where ten diodes are paralleled the failure of a single diode protecting fuse would cause an overload of approximately 11.1% in the remaining nine diodes. Design criteria would normally allow for this magnitude of overload and a machine a shutdown would not be required. However, even a single failed fuse, as described above, would necessitate an alarm notification in order that the operator be made aware that the exciter is operating under an abnormal situation and that the remaining nine diodes are operating under an overload condition. Realizing that this type of overload operation of the remaining nine diodes, although not warranting an immediate shutdown, may have a degrading effect on them, the operator may want to schedule a corrective procedure when it next becomes convenient to do so. In the event that more than one diode path becomes inoperative due to fuse or diode failure the operator may want to schedule an immediate orderly shutdown of the equipment in order to avoid serious degradation of the life expectancy of the remaining diodes. Obviously, from the above discussion, when multiple fuses fail in a given diode circuit the overloading effect on the remaining diodes may be so great that their associated fuses immediately fail, removing one complete diode circuit from operation. This type of catastrophic failure would require the immediate emergency shutdown of the system since the exciter could no longer operate properly.
Brushless exciters rotate at very high speeds. Two-pole machines operate at 3600 RPM and four-pole machines operate at 1800 RPM. Since the fuses and diodes are mounted on the circumference of the exciter's rotor, these components can travel at very high linear velocities. These velocities are well beyond a human operator's abilities to inspect the fuses visually or manually. Therefore, some form of automated equipment is required to assist the operator in determining the existence of a failed fuse or diode.
There are devices available to aid the operator in this function. One such device is disclosed in Mann et al., U.S. Pat. No. 3,866,196 which utilizes a projection tile that, upon a fuse failure, is released by a rupturing wire and centrifugally forced to move outward a short distance and extend radially from the surface of the rotor. The projectile can be later observed through the use of a stroboscopic light or other suitable means of observing the projectile. Another such device is disclosed in Lessmann U.S. Pat. No. 3,030,531 which utilizes an indicator lamp, or other similar means of gaining a visual indication of the operation of each fuse element, connected in parallel to each fuse. Upon failure of a fuse, its associated lamp is subjected to a portion of the current that is no longer able to flow through the failed fuse and the activated lamp can be visually sensed by means of a stroboscopic light or other suitable means. This Lessmann patent further contemplates the use of a transparent fuse that can be stroboscopically inspected or a relatively thin fuse wire that, like the lamp described above, is connected in parallel with each fuse. Upon the failure of a fuse, the subsequent current flowing through its associated fuse wire is sufficient to burn it apart and this parted condition is discernible by use of a stroboscopic light or similar means.
Failure of the diode circuitry can occur in ways that are not detectable by the means discussed above. These include a mechanical or electrical failure that results in an open circuit in series with a diode and its corresponding fuse. This type of failure would result in the fuse, although still in working order, having no current flowing through it. Under this failure mode the Mann projectile would not be released since a current is required to melt a retaining wire for that operation. Also, under this mode of failure, the Lessmann lamp or transparent fuse would not be subjected to the current that is required for their correct operation.
Another area where improvement can be realized is in the prevention of invalid fuse failures. These can occur when the fuse failure indicator itself fails. For example, if the projectile retaining wire in the Mann invention ruptures due to the high centrifugal forces encountered during operation, a failed fuse indication will be evidenced even though the fuse itself is operating satisfactorily. Although this type of false alarming failure is annoying and possibly costly as measured in wasted effort, a more disastrous situation occurs when the failed fuse indicator fails to operate when it should. This could happen, for example, if the lamp in the Lessmann device fails to produce light because of a broken lamp element or the Mann projectile jams or is not released due to some failure in the release wire circuit.
Furthermore, the need for a deliberate manual inspection of the failed fuse detectors described above and the requirement that the failed fuse detector be an element of the design of the rotor itself make possible a significant improvement by their elimination.